As I teeter on the 225-foot-high railroad trestle, a vigorous wind laps at my back. The bridge ends abruptly right here where I'm standing, about 600 feet across the 2,000-foot-wide Kinzua Gorge in northwestern Pennsylvania. Gingerly, I scoot toward the edge of the bridge without lifting my feet and peek over the railing at the end of the trestle, not really wanting to look down.But I have to. That's why I'm here: to see this bridge to nowhere.For more than a century, this was the Kinzua Viaduct, once the longest and highest railroad bridge in the world. Now it's an outdoor observation walkway called the Kinzua Bridge Skywalk.